The present invention relates to a method and device for measuring vertical movements or the vertical component of these movements in the absence of a reference point.
The method of the invention finds applications more particularly in the marine field for measuring, for example, the movements of any floating object subjected to the motion of the waves. This type of measurement is necessary, for example, when using ships which, for one reason or another, are attached to the sea bed and where it is desirable to remove the connection means from the influence of heaving movements. This is the case of ships supporting, for example, an oil drilling string.
The simplest method of measuring vertical movements is that in which a line is stretched between a fixed position with respect to the bottom of the water and a position on the object, and measuring the variations of length of the line between the two points. This method has however drawbacks because the line to be stretched is very long or else it hinders the manoeuvres.
In, for example, French patent No. 2091906, a method for positioning ships as proposed wherein an acoustic telemetry system comprising several transducers located, for example, in a fixed position with respect to the bottom of the water and transmitting pulses which are received by one or more sensors connected to the moving object. The measurement of the variations of the propagation time of the acoustic pulses makes is possible to calculate the variations of distance between the acoustic wave transmitters and receivers and so the movement of the moving object. But acoustic telemetry methods can only be used for measuring very slow movements because of the propagation time, which is sometimes considerable, of the acoustic waves.
In, for example, U.S. Pat. No. 4,515,013, a method of measuring the movements of an object, without material connection with respect to a fixed reference point, is proposed which consists in measuring the accelerations undergone by the object along one or more axes orthogonal to each other and particularly along the vertical axis and calculating, by integration, the movements of the object which result therefrom.
Although the measurement of the vertical movements alone counts for a considered application, a single accelerometer may also associated with a platform connected to the object, with this platform being stabilized as far as the trim is concerned, either by inertia or by pendular suspension.
But in all cases, the measurement methods using accelerometers are relatively costly.
The method and device of the invention avoid the above mentioned drawbacks.